The present application claim priority from French Patent Application No. 01 11356, filed Sep. 3, 2001, the disclosure of which is hereby incorporated by reference.
1. Field of the Invention
The invention relates to a reference current generator that is particularly useful for integrated circuits using low supply voltages. A generator according to the invention produces a current independent of the supply voltage.
2. Description of the Prior Art
To create currents independent of the power voltage, there are known ways of using bootstrap type reference current generators, a simplified example of which is shown in FIG. 1.
The generator of FIG. 1 comprises two P type transistors T1 and T2, two N type transistors T3 and T4 and a resistor R1. The drain of the transistor T1 and the drain of the transistor T3 are connected together; a supply voltage VDD is applied to the source of the transistor T1 and a reference voltage VSS is applied to the source of the transistor T3. The source of the transistor T2 is connected to the source of the transistor T1, and the gate and the drain of T2 are connected together to the gate of T1 and to the drain of T4. Finally, a pole of the resistor R1 is connected to the source of T4 and the reference voltage VSS is applied to another pole of the resistor R1.
The generator of FIG. 1 works as follows. Currents I1 and I2, respectively, cross the transistors T1 and T2, which form a current mirror. The currents I1 and I2 are proportional to each other or, possibly, equal: I1=a*I2.
The current I1 crosses the transistor T3, imposing a voltage VTN3 between the gate and the source of T3; where VTN3 is the threshold voltage of the transistor T3, and is independent of the supply voltage VDD.
The current I2 crosses the resistor R1 and a voltage R1*I2 appears across its terminals. Since the resistor R1 is connected between the gate and the source of the transistor T2, at equilibrium, we have R1*I2=VTN3 giving:
I2=VTN3/R1.
The current I2 is thus independent of the supply voltage VDD, as it depends only on the threshold voltage of the transistor T3 and the resistor R1.
The current I2 obtained may be copied for other uses. For example, it may be copied by means of a copying transistor T5, whose gate and source are respectively connected to the poles of the resistor R1. The drain of the transistor T5 is connected to the ancillary circuit which uses the reference current flowing in the transistor T5. The reference current is directly proportional to the current I2 flowing in the resistor R1.
It will be noted that the current I2, while independent of the supply voltage VDD, is on the contrary dependent on the temperature of the circuit because the threshold voltage VTN3 is itself linearly dependent on the temperature. We have:
I2=(VTN3(T0)xe2x88x92K(Txe2x88x92T0))/R1, with
T being the temperature;
T0 being a reference temperature; and
VTN3(T0) being the threshold voltage of T3 at the temperature T0.
The variation, as a function of the temperature, of the current produced by a generator is not necessarily a drawback. Indeed, certain circuits use reference currents whose value is variable as a function of the temperature.
If not, it is fairly easy to accept a variable current such as the one produced by a generator according to FIG. 1, inasmuch as the variations of the threshold voltage VTN3 as a function of the temperature T are known and are, furthermore, simple: the threshold voltage VTN3, and therefore the current I2 that crosses the resistor R1, varies linearly as a function of the temperature: I2 is indeed equal to I2=I0*(Ixe2x88x92b*T).
If a constant current is necessary, there are known ways of combining a generator that produces an I=I0*(1+b*T) type current with a generator producing an I=I0*(1xe2x88x92b*T) type current to obtain a current independent of the temperature.
To create currents, there are also known ways of using reference current generators that use a bipolar transistor. A simplified example of a reference generator of this kind is shown in FIG. 2.
As compared with the generator of FIG. 1, the circuit of FIG. 2 additionally comprises a bipolar transistor T6. An emitter of the transistor T6 is connected to the source of T3 and the reference voltage VSS is applied to a collector and a base of T6 which are connected together. Finally, the gate of T3 is no longer connected to the source of T4 but to its gate.
The generator of FIG. 2 works similarly to FIG. 1. The current I2 flowing in the resistor R1 is simply equal in this case to:
I2=VBE6/R1,
VBE6 being a threshold voltage between the base and the emitter of the transistor T6 and being independent of the supply voltage VDD. On the contrary, VBE6 depends on the temperature linearly.
Additional information on the making of generators such as those shown in a diagrammatic view in FIG. 1 or FIG. 2 may be found in the document: xe2x80x9cCMOS Analog Circuit Designxe2x80x9d, Editions Holt Rinehart and Winston 1987.
The generators according to FIG. 1 or FIG. 2 are used whenever it is desired to obtain a reference current independent of the supply voltage. This need arises frequently because the supply voltage of a circuit can often vary. Indeed, this voltage often depends on the power given to the circuit.
However, the generators according to FIG. 1 or FIG. 2 have a major drawback related to the value of the minimum supply voltage VDDMin to be used to supply such generators. Indeed, the supply voltage VDD applied must be sufficient to turn on or even saturate all the transistors of the generators, so that a current flows in these transistors.
For example, for the generator of FIG. 1, the minimum voltage VDDmin to be applied is equal to:
VDDmin=VTN3+VDS4+VGS2, with:
VTN3, threshold voltage of T3, on the order of 0.60 V, and
VDS4, voltage between the drain and the source of the transistor T4, on the order of 0.15 V, and
VGS2, voltage between the gate (or the drain, since they are connected together) and the source of T2, on the order of 0.70 V.
Consequently, the voltage VDDmin for the circuit of FIG. 1 is on the order of 1.5 V.
In the same way and for the same reasons, for the circuit of FIG. 2, the minimum supply voltage VDDmin to be used is equal to:
VDDmin=VBE5+VGS3+VDS1, with:
VBE5, voltage between the emitter and the base of T5, on the order of 0.7 V,
VGS3, voltage between the gate and the source of the transistor T3, on the order of 0.65 V, and
VDS1, voltage between the drain and the source of T1, on the order of 0.15 V.
Consequently, the voltage VDDmin necessary to power the circuit of FIG. 2 is on the order of 1.5 V.
Thus, whatever the known current generator used, the minimum supply voltage VDDmin to be used is on the order of 1.5 V.
Now, a minimum voltage of this kind may be prohibitive, especially for circuits made by means of the smallest submicron technologies, for example technologies at the 0.25 xcexcm level or below, which can use only voltages lower than 1.5 V, or even 1.2 V for 0.13 xcexcm technologies.
The present invention relates to a current generator for the production of a reference current.
According to an embodiment of the invention, the generator comprises a first P type transistor, a source of which is connected to a first pole of a resistor and a gate of which is connected to a second pole of the resistor, the reference current, flowing in the resistor, being variable as a function of a threshold voltage of the first transistor, and a second N type transistor, having a drain, a gate and a source connected respectively to the second pole of the resistor, the first pole of the resistor and the drain of the first resistor, the second transistor working in saturation mode.
The reference current produced is thus fixed by the voltage between a gate and a source of the first transistor, which is itself equal to the threshold voltage of the transistor. The reference current produced therefore does not depend on the supply voltage.
In an embodiment of the invention, the above generator is advantageously supplemented by a current source comprising a first pole to which a supply voltage is applied and a second pole connected to the first pole of the resistor.
The current source that is used provides current and power to the resistor and the first transistor. In particular, it gives the reference current flowing in the resistor and the current in the first transistor.
The first transistor and the second transistor are chosen so as to be adequately sized (in terms of gate length/width) so that they are saturated in normal operation of the generator. Thus, as shall be seen further below, the current flowing in the transistor is very low and a current given by the current source is very close to the reference current produced by the current generator according to the invention.
In an embodiment of the invention, a reference current is applied to the drain of the first transistor.
For the generator according to an embodiment of the invention, the minimum supply voltage to be applied is equal to the sum of the voltage between the gate and the source of the second transistor and the voltage between the poles of the current source. It is therefore lower (in the range of 1 to 1.2 V) than the voltage applied for known generators, as shall be seen more clearly here below in the examples.
The uses to which a current generator according to the invention can be put therefore includes the making of circuits in the finer technologies having a low supply voltage.
The generator according to an embodiment of the invention is advantageously supplemented by a third transistor, which is an N type transistor, having a gate and a source connected respectively to the gate and the source of the second transistor.
The third transistor and the second transistor thus form a current mirror: the third transistor copies the reference current flowing in the second transistor and a current proportional (or equal) to the reference current (and therefore independent of the supply voltage) is thus accessible at the drain of the third transistor and may be used by an external circuit.
According to an embodiment of the invention, the current source used in the generator comprises a fourth transistor and a fifth transistor, the supply voltage being applied to the common source of the fourth transistor and of the fifth transistor, the gate of the fourth transistor and the gate of the fifth transistor being connected together to the drain of fifth transistor and to the drain of the third transistor and the drain of the fourth transistor being connected to the first pole of the resistor.
In accordance with an embodiment of the invention, the current generator includes a first node to which an input current whose value is dependent on supply voltage is applied, the first node dividing the input current into a first and second current. A first transistor is connected to the first node and operates to pass the first current. A resistor is connected to the first node and passes the second current. In this configuration, the value of the second current is independent of supply voltage and is approximately equal to a ratio of a threshold voltage of the first transistor to a resistance of the resistor.